The present invention relates to a continuous process for the preparation of foamed plastics material which is formed by reaction of diisocyanates or polyisocyanates with aminoplast precondensates and exhibits improved fire resistance, and to an apparatus for carrying out the process.
Processes for the preparation of aminoplast precondensates with the aim of combining the good properties of the two types of plastics materials, that is to say the aminoplasts and the polyurethane plastics, with one another are already known.
Thus, for example, U.S. Pat. No. 3,632,531 discloses the preparation of polyurethane foams by the condensation of organic polyisocyanates with so-called "methylol resins", that is to say aminoplast precondensates, under the catalytic action of organic tin salts and secondary or tertiary amines. The disadvantage is that the water content of the reaction system in this process and in the other similar processes known hitherto should be far below 10% and this means that industrially available aminoplast precondensate solutions which contain considerable amounts of water, have first to be evaporated before they are used. Furthermore, after-curing is necessary in order to achieve optimum flame resistance and the necessary mechanical strength. On the other hand, acid compounds are regarded as condensation retarders for polyurethanes (see for example, Kunststoff-Handbuch (Plastics Handbook) Carl Hanser-Verlag, volume x, 1966, page 151).
West German Offenlegungschrift No. 1,918,181 discloses the preparation of foams from urea/formaldehyde or melamine/formaldehyde resins, and particularly phenol/formaldehyde resins by reaction with isocyanates in two stages; the formaldehyde resin first being produced in the presence of an acid catalyst and in the presence of a blowing agent, such as a fluorinated hydrocarbon, and the isocyanate being added only subsequently. Since, as is known, urea/formaldehyde, melamine/formaldehyde and phenol/formaldehyde resins rapidly undergo complete condensation in an acid medium, only a few free hydroxy groups are still available for the reaction with the isocyanate in a process of this type, so that uniform distribution of the formaldehyde resin and polyurethane elements in the condensation product is not achieved.
United Kingdom Pat. No. 1,541,776 discloses that it is possible to prepare, from an aminoplast precondensate and a polyisocyanate, light foams which, apart from the carbon dioxide supplied by the reaction of the polyisocyanate with water, require no further foaming agent, if the aminoplast component is employed as an aqueous solution of a water-soluble precondensate, the amount of polyisocyanate is so chosen that both a significant condensation reaction takes place with the precondensate and an optimum blowing reaction takes place with water and if certain combinations of acid and basic or metal-organic catalysts are employed, it being necessary to carefully control the addition of the latter. In this way it is possible to produce foams which have good combustion and insulating characteristics, and which cure tack-free without further heat after-treatment, in an economical manner, in a single process step and with short production times.
Accordingly, United Kingdom Pat. No. 1,541,776 claims a process for the preparation of a flame-resistant foamed plastics material which comprises condensing a water-soluble aminoplast precondensate of formaldehyde and urea and/or melamine, containing at least one N-methylol group and having a water content of 3 to 50% by weight, with an organic isocyanate, which is at least bi-functional, in the presence of 0.5 to 5.0% by weight relative to the foamable mixture of an acidic curing catalyst and in the presence of an organic amine as basic catalyst and optionally additionally an organic metal compound which also has a catalytic action, the said aminoplast precondensate being added to the organic isocyanate, to which the amine or amine and organic metal compound has previously been added, at the same time as, but separately from the acid curing catalyst, the ratio of water to organic isocyanate being from 0.05 to 4.0:1 parts by weight.
The process disclosed in United Kingdom Pat. No. 1,541,776 may be carried out discontinuously very successfully. However, difficulties arise when it is attempted to carry out the process continuously under optimum conditions.
The difficulties are essentially concerned with the fact that the preparation of the pre-adduct of the organic polyisocyanate with the basic catalyst (above all with the amines containing hydroxyl groups) is extremely difficult because of the high rate of reaction, the high heat of reaction and the associated rapid rise in viscosity of the pre-adduct formed. As is well-known, in the case of polyurethane foams the basic catalyst is first added to the polyol and is only then, diluted in this manner, added to the organic isocyanate. Because of the special nature of the process according to United Kingdom Pat. No. 1,541,776 the basic catalyst must be added directly to the organic polyisocyanate and in particular must necessarily be added in extremely high concentrations of up to 14% by weight, relative to the polyisocyanate. This may result in premature gelling of the pre-adduct formed, which then prevents continuous production of the foams because the conventional foaming apparatus rapidly becomes blocked. As a result, trouble-free production of a satisfactory product becomes impossible.
Surprisingly, it has now been found that the disadvantages described above can be avoided if the process is carried out in two defined stages and the polyisocyanate and the basic catalyst and organic metal compound are brought together in the first mixing stage at quite specific temperatures and with specific residence times.